1. Field of the Invention
The present invention relates to an image forming apparatus and a control method therefor, and more particularly to a technique of controlling image formation timing in image formation performed using an intermediate transfer member.
2. Description of the Related Art
Conventionally, image forming apparatuses, such as copying machines and printers, utilizing electrophotography include ones which perform image formation using a belt-like intermediate transfer member (hereinafter referred to as “the intermediate transfer belt”). In general, in an image forming apparatus of this type, electrostatic latent images formed on respective photosensitive members are developed by toner to be visualized, and the toner images are transferred onto the intermediate transfer belt, followed by being collectively transferred onto a recording material (such as a sheet). This transfer method is generally called the intermediate transfer method.
In a full-color image forming apparatus employing the above-mentioned intermediate transfer method, toner images in respective different colors, such as four colors of Y (yellow), M (magenta), C (cyan), and K (black), are transferred onto an intermediate transfer belt in a manner superimposed one upon another to thereby form a color image. In one method for achieving superimposition of colors with high accuracy to thereby obtain an excellent image, an HP mark indicative of a reference position (hereinafter referred to as “the HP (home position)”) is marked on an intermediate transfer belt for detection by a sensor, and timing for starting color-specific image formation is determined based on time at which the HP mark is detected.
FIG. 11 is a schematic view of an intermediate transfer member. As shown in FIG. 11, on the inner peripheral surface of the intermediate transfer member 205, there are marked a first HP mark 1101 and a second HP mark 1102 each indicative of a reference positions (HP) at respective locations diametrically opposed to each other. Further, within the intermediate transfer member 205, there is disposed a mark sensor 1103 that detects the first HP mark 1101 and the second HP mark 1102.
As the intermediate transfer member 205 is rotated in a direction indicated by an arrow in FIG. 11, the mark sensor 1103 detects each of the first HP mark 1101 and the second HP mark 1102 once whenever the intermediate transfer member 205 performs one rotation.
In the case of forming a full-color image by this image forming apparatus, when the mark sensor 1103 detects the first HP mark 1101 after the apparatus has entered a state capable of image formation, a first-page yellow image forming process (formation of a latent image, development of the latent image by toner, and primary transfer of the toner image onto the intermediate transfer member (ditto for the other colors)) is started. Then, when the mark sensor 1103 detects the second HP mark 1102, a second-page yellow image forming process is started.
Next, when the mark sensor 1103 detects the first HP mark 1101, a first-page magenta image forming process is started, and when the mark sensor 1103 detects the second HP mark 1102, a second-page magenta image second-page magenta image forming process is started. Further, cyan image forming processes and black image forming processes are sequentially carried out in the same manner as described above, whereby first-page and second-page toner images in the respective colors are respectively primarily transferred onto the intermediate transfer member in superimposed relation.
That is, during a time period between detection of the first HP mark on the intermediate transfer member and detection of the second HP mark on the same, a first-page toner image in a color is formed on the intermediate transfer member, and during a time period between detection of the second HP mark and detection of the first HP mark, a second-page toner image in the color is formed on the intermediate transfer member.
In this type of image forming apparatus, images corresponding to respective two pages are formed with reference to time of detection of the first HP mark and the second HP mark, as described above. For this reason, when at least one of the HP marks is stained or peeled off and HP mark cannot be normally detected, excellent image formation is made impossible. To solve this problem, an image forming apparatus has been proposed in which intervals (cycles) at which the mark sensor 1103 has actually detected the first HP mark 1101 and the second HP mark 1102 are measured, and when a detection interval is not normal, a notification of the abnormality is issued so as to stop image forming operation (see Japanese Laid-Open Patent Publication (Kokai) H09-054504). Further, in addition to the apparatuses that perform image formation timing control in the above-described manner, there has also been proposed an apparatus that forms, when one of a first HP mark and a second HP mark is determined to be abnormal, an image without using the one HP mark which is determined to be abnormal but using only the other HP mark which is determined to be normal (see Japanese Laid-Open Patent Publication (Kokai) 2000-66562).
In the above described abnormality-detecting method, however, when an abnormality is detected, it is required to stop the operation of the image forming apparatus and carry out maintenance operation, such as HP mark cleaning or replacement of the intermediate transfer member, and hence image forming operation cannot be carried out until completion of the maintenance operation, which leads to considerable reduction of productivity of the image forming apparatus.
Further, when one of the HP marks becomes abnormal, one page of an image is formed based on detection of the normal HP mark, but the other page of an image is not formed, causing degradation in productivity. Moreover, when the two marks become abnormal, the image forming apparatus is stopped so that image formation output cannot be obtained. This is inconvenient to users when they are in haste to obtain image formation output, e.g. when it is required to output a small number of copies urgently. On the other hand, one of factors causing reduction of productivity is that it takes a longer time period to detect a first HP mark depending on the position where the intermediate transfer member has been stopped.
More specifically, according to the method of determining image formation start timing based on HP mark detection, a time period taken before the start of the present sequence of image forming processes (electrostatic latent image forming operation) varies with the distance (positional deviation) between the sensor and an HP mark positioned when the intermediate transfer belt is stopped upon termination of the immediately preceding sequence of image forming processes. As shown in FIG. 23A, for example, when the intermediate transfer belt has been stopped in a position where the HP mark is to be detected by the sensor immediately after stabilization of rotation of an intermediate transfer belt-driving motor, a time period Ta between the start of motor rotation and the detection of the HP mark is short, so that it is possible to quickly start electrostatic latent image forming operation. On the other hand, as shown in FIG. 23B, when the intermediate transfer belt has been stopped in a position where the HP mark is remote from the sensor, it takes a longer time period Tb before electrostatic latent image forming operation is started. This consequence is inconvenient when users are in haste to obtain image formation output, e.g. when it is required to print a small number of copies urgently.
In other words, in order to reduce first copying time, it is desirable to constantly stop the intermediate transfer belt in an optimum position shown e.g. in FIG. 23A after termination of image formation.
However, since tension is applied to the intermediate transfer belt stretched around a plurality of rollers, if the intermediate transfer belt is constantly stopped in the same position, deformations in portions of the belt in contact with the respective rollers, i.e. so-called permanent deformations occur, which can cause degradation of the intermediate transfer belt and considerably lower image quality.
To solve this problem, an image forming apparatus has been proposed, for example, in which a plurality of timers for counting a time period from detection of an HP mark to stoppage of an intermediate transfer belt are arranged so as to prevent the intermediate transfer belt from stopping in the same position (see Japanese Laid-Open Patent Publication (Kokai) No. 2001-201994). In this case, the intermediate transfer belt is not constantly stopped in an optimum position, and hence first copying time in next image forming operation is sometimes increased.
Further, an image forming apparatus is known in which a plurality of HP marks are marked on an intermediate transfer belt so as to prevent the intermediate transfer belt from being constantly stopped in the same position and enable quick HP mark detection (see Japanese Laid-Open Patent Publication (Kokai) No. H06-289684). However, with this arrangement, it is required, in actuality, to mark numerous HP marks on the intermediate transfer belt so as to detect an HP mark in a minimum wait time while preventing permanent deformation of the intermediate transfer belt, which causes an increase in manufacturing costs.
As described above, in an intermediate transfer type image forming apparatus in which colors are superimposed based on HP mark detection, it is difficult to constantly stop an intermediate transfer belt in the same position upon termination of an image forming process, which causes variation in a time period from a time point when the intermediate transfer belt in stoppage is driven to a time point when an HP mark is detected by a sensor for the first time. For this reason, it sometimes takes time before the start of image formation. Further, to solve this problem, it is necessary to arrange timers, for example, which causes an increase in manufacturing costs. Thus, the time period before the first-time HP mark detection occupies a large portion of a time period required for the entire image forming operation, which is generally inefficient for users who very often uses a job for printing a small number of sheets per one image forming operation.
As a countermeasure therefor, an image forming apparatus has been proposed, in which image formation based on detection of an HP mark on an intermediate transfer belt or image formation based on the circumferential length of the intermediate transfer belt is selectively carried out. In the latter image formation, the circumferential length of the intermediate transfer belt is detected in advance based on the HP mark detection, and an image writing reference position signal for starting image formation is issued to thereby carry out the image formation. When an appropriate number of clocks corresponding to the circumferential length have been counted and hence the intermediate transfer belt has completed one rotation, the next image writing reference position signal is issued for the next image formation (see Japanese Laid-Open Patent Publication (Kokai) No. 2004-240306). With this image forming apparatus capable of selecting an appropriate image formation method, the first copying time can be reduced. If, however, the HP mark detection cannot be carried out, it is no longer possible to detect the circumferential length of the intermediate transfer belt, making it impossible to carry out the image formation based on HP mark detection. As a result, the image forming apparatus stops operating, and an image formation output cannot be obtained. This causes inconveniences for users who are in haste to obtain image formation output.